Conventional fertilizers are commonly derived from petroleum. In fact, a single 40-pound bag contains the equivalent of 2.5 gallons of gasoline.

Animal Ingredients

Organic fertilizers may also contain ingredients that have not been harvested in the most eco-friendly way. Bone, blood and feather meals are byproducts of the meat industries and are added to fertilizers as a source of nutrients. Fish meal, another industry byproduct, may be contaminated with PCBs and mercury and could come from overharvested fish populations. The Northeast Organic Farming Association allows their use only under the strict warning that humans should avoid direct contact with them.

Peat

Peat, another additive in many fertilizers, could actually contribute to global warming. Peat bogs store more carbon dioxide than all of the world's tropical rainforests, and when the peat is harvested, all the stored CO2 is released back into the atmosphere. These wetland areas also support diverse ecosystems that can be damaged when the bogs are drained to harvest peat. Often the drained land is converted into forests, but the wildlife diversity is lost. In some areas, butterfly populations that thrived in wetland peat bogs have decreased as much as 90 percent.

Sewage

Finally, some organic fertilizers contain sewage sludge, leftover from wastewater treatment plants. Sewage sludge comes with all the pharmaceuticals, antibacterials, industrial synthetic chemicals, heavy metals and other chemicals that wastewater treatment plants aren't able to remove. Though the EPA has endorsed sewage sludge, or biosolids, as fertilizers for years, a February 2008 Environmental Science and Technology study revealed that earthworms living in soil treated with sludge were absorbing the pharmaceuticals and personal care product ingredients that treatment plants leave behind. While the study wasn't conclusive, their presence in earthworms led scientists to suspect chemicals were building up in the crops growing in the soil.

Nutrient Pollution

In addition to their oil base, synthetic fertilizers are spiked with concentrated forms of nitrogen and phosphorus. The excess not absorbed by plants runs off into storm drains that feed into rivers and streams, contributing to algae blooms that deprive waterways of oxygen and kill off aquatic life. Such an algal bloom has created a "dead zone" in the Gulf of Mexico the size of New Jersey.

Organic fertilizers also contain nitrogen and phosphorus that can wash off into waterways. And just like conventional fertilizers fortified with synthetic ammonia, manure and guano (bird droppings) both have high levels of naturally occurring ammonia that can "burn" plants.

Microorganisms

Conventional fertilizers can prove detrimental to your lawn in the long run. Healthy soils contain billions of beneficial microorganisms. Synthetic fertilizers can kill these helpful organisms, sterilizing your soil and creating a dependence, like an addiction, to the synthetic replacements.